/Engine/Source/Runtime/InteractiveToolsFramework/Public/InteractiveTool.h
Indexof the watcher, which can be used in SilentUpdateWatcherAtIndex
template<typename PropType> int32 WatchProperty ( const PropType & ValueIn, typename [TPropertyWatcher](API\Runtime\InteractiveToolsFramework\FWatchablePropertySet\TPropertyWatcher)< PropType >::FChangedCallback OnChangedIn )